I've seen similar restrictions on several sites. I don't think it has to do with pollution, since using a filter doesn't add anything to the water. My guess is that since filters won't remove everything, they don't want people to have a false sense of security, or some other idiotic logic about protecting people from themselves.
Whatever the reason, it's absurd, I don't know what campers in Iowa are expecteed to due. I'd imagine any serious outdoor enthusiest in Iowa simply crosses the border to buy what they need.